The invention relates, generally, to cryogenic vessels and, more particularly, to an improved control regulator and delivery system therefore.
Cryogenic vessels typically consist of an insulated double-walled tank for storing fluids such as liquid oxygen nitrogen or argon at very low temperatures. The fluid is stored in the tank as a liquid and is dispensed therefrom and used as a gas. For example, liquid oxygen can be dispensed from the tank in a gaseous state for use in medical or industrial applications.
While the cryogenic vessel is insulated, it is impossible to prevent all heat transfer between the interior of the tank and the external environment. As a result, the oxygen, although introduced into the tank in a liquid state, will slowly but continuously vaporize creating a gaseous head of oxygen above the liquid body.
Because the oxygen is used in a gaseous state, it is advantageous to use this head gas before vaporizing the liquid supply. If the head gas were not used it would eventually have to be vented from the tank to maintain the tank's internal pressure. Therefore, by using the head gas first, the waste of oxygen resulting from the venting procedure is eliminated.
One problem with supplying the oxygen directly from the head is that the head pressure will not always be sufficient to meet use requirements. When the head pressure is insufficient, the liquid body must be vaporized to meet the use requirements and to rebuild the head pressure.
To coordinate the supply of oxygen from the gas head and liquid body, a relatively complex delivery system has been developed. The known delivery system is illustrated in FIGS. 1 and 2 and consists of a double-walled cryogenic storage tank 1 holding a supply of fluid, for example, liquid oxygen, shown at 2. A gaseous head 3 of vaporized oxygen will form due to the transfer of heat between the interior of the tank and the external environment. A fluid supply line 5, having a control valve 7, is provided for delivering liquid oxygen to tank 1.
To dispense the liquid oxygen from the tank for use, a pressure building coil 9 is connected to the bottom of tank 1. The liquid oxygen is free to flow from tank 1 into coil 9. Because pressure building coil 9 is disposed relatively close to the exterior of tank 1, heat transfer between the external environment and the liquid in coil 9 will be relatively great. As a result, the liquid oxygen will be vaporized such that the pressure in coil 9 will build. Coil 9 is connected to pressure building line 8 which includes a pressure building regulator 13, a pressure building valve 11 and a return line 23 the functions of which will be hereinafter described.
A vaporizer line 10 connects pressure building line 8 to vaporizer 17 and includes an economizer regulator 15. Vaporizer 17 is connected to gas use line 19 and is disposed closely adjacent to the external wall of tank 1 such that heat transfer in vaporizer 17 will be great enough to vaporize any remaining liquid oxygen before it is delivered to gas use line 19. Gas use line 19 includes a gas use valve 21 for controlling the delivery of gas for its intended use.
Return line 23 connects the gas head 3 in tank 1 to the pressure building line 8 and vaporizer line 10. Finally, a vent system 24 is connected to pressure building line 8 and vaporizer line 10 and consists of a vent valve 25, pressure gauge 26, and pressure control valve 27. Gas can be vented from the tank 1 when the pressure of head 3 rises above a predetermined limit. Burst disc 28 ensures that under extreme conditions tank 1 and the other components will not be damaged from an abnormally large pressure build up.
To describe operation of this system, assume that a supply of liquid oxygen 2 is in tank 1 and that a gas head 3 has formed. Also assume that the pressure in the system is lower than the predetermined value set by pressure control valve 27 such that no gas is being vented from the system.
To dispense oxygen gas, gas use valve 21 is opened and head gas is delivered via use line 19. As long as the pressure of head 3 is sufficiently great, i.e. above the value set at economizer regulator 15, pressure building regulator 13 will be closed such that gas is provided directly from head 3 through economizer line 23 to vaporizer line 10. From vaporizer line 10 the gas passes through vaporizer 17 and is delivered for use from gas use line 19.
If the pressure of head 3 falls below the predetermined value set at economizer regulator 15, pressure building regulator 13 will open such that liquid oxygen is removed from the bottom of tank 3 and enters pressure building (vaporizer) coil 9 where it is vaporized and delivered to pressure building line 8. Note, pressure building regulator 13 is generally set at a pressure slightly below economizer regulator 15 so that the two are related, but not overlapping. If valve 11 is open, gas will flow to the junction 20 of pressure building valve 13, economizer valve 15 and return line 23. Because economizer valve 15 is closed, gas will enter head space 3 via line 23 to build and maintain tank pressure.
To supply oxygen gas at valve 21 when the head pressure is lower than the setting of economizer regulator 15, liquid oxygen is withdrawn from tube 5 and enters vaporizer coil 17. The liquid oxygen is converted to gaseous oxygen and delivered to valve 21 via use line 19.
While this system allows the gas to be taken from the head 3 as long as there is sufficient head pressure, it requires two regulators and relatively complicated plumbing, as best shown in FIG. 2. As a result this system is expensive to manufacture and maintain.
Thus, a delivery system for a cryogenic vessel that is of a simple design and is relatively inexpensive to manufacture and maintain is desired.